Spectrally selective optical couplers, also known as channel drop or add filters, are utilised for extraction of a single wavelength channel from a broadband optical signal, or for insertion of a single wavelength channel into a broadband optical signal. Typically, spectrally selective couplers are used in wavelength division multiplexed optical communications systems for adding and dropping a single wavelength channel.
Channel drop filters have previously been implemented as dual-waveguide couplers. The article “Narrow-Band Optical Channel-Dropping Filter”, Journal of Lightwave Technology, vol. 10, no. 1, January 1992 (Haus et al.) describes an optical channel-dropping filter comprising a first and a second waveguide, the first of which contains a λ/4 shifted distributed feedback (DFB) resonator. Light propagating in the second waveguide is coupled to the first waveguide by evanescent coupling between the two waveguides. Only one wavelength of light is resonant in the first waveguide, and consequently only that wavelength of light is efficiently coupled to the first waveguide. By making the λ/4 shifted DFB resonator asymmetric (i.e. the grating is longer on one side of the λ/4 shift), light can be coupled out of the DFB resonator.
However, prior art channel drop filters have some significant drawbacks and limitations. The filters are difficult to manufacture, due to the fact that very precise placement of the waveguides is required, in order to obtain a reliable evanescent coupling. Furthermore, the prior art filters are difficult to control and tune. The coupling strength and the coupled wavelength is, to a large extent, fixed once the device is assembled. Also, each filter needs to be of a certain size in order for the desired discrimination to be achieved. In particular, when a number of channels are to be dropped separately (e.g. when constructing a demultiplexer), the device needs to be quite large. Yet another problem with the prior art filters is that they are difficult to implement in a fibre configuration, since the evanescent coupling between the waveguides needs to be very accurate. Any perturbation of either of the waveguides can cause large uncontrolled changes in performance.
WO 02/06878 discloses a spectrally selective coupler having a new geometry and a new principle of action compared to yet earlier couplers. It is proposed in that publication to couple light transversally out from an optical fiber by means of a deflector in the form of blazed optical Bragg gratings. Light is coupled transversally from the fiber into an external resonator of the Fabry-Perot type. The light wavelength that is resonant in the external resonator is more strongly coupled to and from the optical fiber. By altering the properties of the external resonator, such as the distance between the reflecting surfaces, the wavelength to be coupled can be tuned.
Although the technology disclosed in WO 02/06878 provided some important improvements over the yet earlier art, some problems still remain. The technology has an inherent geometrical problem, in that the separation between the mirrors of the external resonator must be about 20 μm or less in order to allow convenient tuning. A larger separation gives a shorter free spectral range, and hence a smaller separation between the resonant wavelengths. It turns out that it becomes difficult to arrange two optical fibers within the same external resonator, when coupling from one fiber to another is to be accomplished. This is not only due to the limited space between the reflected surfaces of the resonator, but also due to the fact that both fibers have to be accurately positioned inside the resonator with respect to the two reflective surfaces of the resonator. Moreover, since the resonant mode of the external resonator overlaps spatially with the deflector in the optical fiber, there will be interactions between the light in the external resonator and the light still confined to the core of the fiber. These interactions might-disturb the confined light, which propagates inside the core of the fiber.